Green building materials yield a two-fold answer, says S.Ananthanarayanan.
The crisis of climate change that the world faces calls for urgent solar, wind or tidal energy, and improving efficiency of traditional processes are very well, but, as Greta Thunberg says, our house is on fire! We need measures that bite large chunks out of the carbon economy, not ways that inch towards uncertain targets.
Galina Churkina, Alan Organschi, Christopher P. O. Reyer, Andrew Ruff, Kira Vinke, Zhu Liu, Barbara K. Reck , T. E. Graedel and Hans Joachim Schellnhuber, from Yale University, the Potsdam Institute, Gray Organschi Architecture (architects in New Haven) and Tsinghua University, Beijing, present, in the journal, Nature Sustainability, a strategy that could do just this. The solution they present is to replace cement and steel, which consume large energy to produce, with wood, in building construction. On the one hand, the move would eliminate one of the largest sources of atmospheric CO2. On the other, regrowing trees, which would be felled to provide building material, would draw carbon out of the atmosphere ‑ as good as storing carbon in the buildings where the forest has moved!/font>
Apart from the heat required to produce cement and steel, the very processes of production are sources of CO2 emission. The main component of cement is lime, which is produced by the reduction of limestone, or calcium carbonate, by heat, in kilns. The reaction is like this:
We can see that along with lime, CO2 is produced. And in good quantity, 44 parts, by weight, of CO2 for 56 parts of lime.
When it comes to iron from its ore, the reaction is like this:
The process produces 132 parts, by weight, of CO2 for 112 parts of iron. The carbon monoxide (CO) comes from coal and the production of steel is one of the largest sources of CO2 in the atmosphere.
The automobile and use of coal for power and in industry, of course, are the main causes of rising CO2. The use of concrete, which consists largely of cement and steel, has been the other major contributor. With plentiful energy and food supply (thanks to chemical fertilizer, whose manufacture emits huge CO2), the last century has seen unprecedented population rise and building activity. And construction, during the last century has been more and more with concrete.
The paper in Nature Sustainability points out that vegetation, which stores carbon, cannot, by itself, work as a carbon sequester, because microorganisms break vegetable matter down and put carbon back in the atmosphere. The great storage of carbon, as coal and oil, in ages past, was perhaps because present-day microbes had not evolved, and thanks to conditions that buried and compressed organic matter. As microbes are active now, burying trees would not work today, but using wood as the material for building construction could still achieve the same end. Why find methods to capture and confine CO2, with the attendant risks, when we have the job done in giant trees? We only need to keep these trees from decomposing – our buildings would become the carbon stores - and we would block the CO2 that comes from cement and steel manufacture too, the paper suggests.
Wood, which was the dominant building material in the past, has limits to the crushing loads it can bear. This puts a cap on how high a wooden structure can get. Tall buildings were hence of masonry, and with the advent of cement, of concrete, which is cement with sand and rubble. As concrete, which has compressive strength, cannot take bending loads, the load-bearing beams in buildings were of wood, or of steel. But it was found that concrete could be strengthened with steel bars. And, with the additional advantage of uniformity and specific shapes being possible, very strong structures are now built using a framework of reinforced concrete.
“The buildings and construction sector currently accounts for about half of all global steel demand,” the paper says. There could be marginal improvements in efficiency, and reuse of scrap steel is only a partial solution. Yet another cost of using concrete is extracting sand from beaches, rivers and the sea, and the environmental effect of mining. The result is reduction in the capacity of water bodies to absorb CO2, as well as loss of forests, the paper says.
Mass timber that is now available, the paper says, consists of specifically designed structural components that are laminated using smaller board, as glue-laminated (gluelam) beams or cross-laminated timber (CLT) panels. Methods of fabrication and use, with adhesives and mechanical fasteners have made it possible to build on the strengths of wood as a building material, even fire-resistant (see box) buildings 18-storeys tall have been found practical, the paper says. “…engineered timber products and structural systems offer a potential substitute for much of the mineral-based materials in urban building construction,” the paper says.
Analyses by the authors suggest that using timber for new urban housing has carbon storing potential of between 0.01 to 0.68 billion tons a year, depending on how far timber replaces concrete, and how the dwellings are designed. For comparison, the carbon absorbed by the earth’s whole biosphere is 2.3 billion tons in a year. Over 30 years, if half the new constructions used wood, the paper says, between 1 billion to 1.1 billion tons of CO2 would get stored.
As for the supply of wood, the paper finds that until 50% of construction becomes wood-based, almost all the wood needed could be met from just part of the wood being drawn for short-term purposes, without affecting the net harvest from the forests. The paper cites an FAO study that says out of 65 countries that were evaluated, 43 countries harvested less wood than the forests grew. And overharvesting, where there was any, has been declining. There is hence wood to meet the demand even with 90% of new constructions being wood based, at the current average floor area. And then, materials like bamboo and other plant fibres could supplement wood resources.
The paper emphasizes that a pre-condition to harvest and transfer carbon to the city is that we maintain forest sustainability and continue reforestation. This would need legal and political commitment. Forest dwellers need to be empowered.
“The history of transcultural familiarity with wood and plant based construction material and assemblies, especially in Asia, Oceania and Africa, suggests an alternative future for buildings. In a few decades, a material revolution, scaled in its application to global urbanization and to the sustainable capacities of its forest sources, may balance material supply, material demand and environmental burdens and benefits, while answering the challenge of urgent climate action,” the paper says.